Caissons

Groundhog is slang for brakemen who work with caissons,

which are constructed such that the water can be pumped out, keeping the working environment dry.

[Schematic cross section of a pressurized caisson]

Shallow caissons may be open to the air, whereas pneumatic caissons, which penetrate soft mud, are sealed at the top and filled with compressed air to keep water and mud out at depth. An airlock allows access to the chamber. Workers move mud and rock debris (called muck) from the edge of the workspace to a water filled pit, connected by a tube (called the muck tube) to the surface. A crane at the surface removes the soil with a clamshell bucket.